The invention relates to high-temperature heat treatment of carbon bodies containing sodium, and more particularly to treating the gaseous effluents produced during the heat treatment.
A particular field of application for the invention is making carbon fiber fabrics or preforms to constitute fiber reinforcement for composite material parts such as carbon/resin composite parts, e.g. C/epoxy or C/phenolic resin parts, or thermostructural composite parts, such as carbon/carbon (C/C) composite parts or carbon-reinforced ceramic matrix composite parts.
Such fiber fabrics are conventionally obtained using carbon-precursor fibers since they are better at withstanding the textile manufacturing operations required for forming fabrics than are carbon fibers. Carbon-precursor fibers in common use are preoxidized polyacrylonitrile (PAN) fibers, fibers made of pitch, phenolic resin fibers, and rayon fibers.
In certain applications at least, it is necessary not only to transform the precursor into carbon, but also to perform subsequent heat treatment at high temperature, typically above 1000° C., and under low pressure, for the purpose of eliminating metals or metallic impurities, in particular sodium coming from the precursor, and/or in order to impart particular physico-chemical properties to the fibers.
Thus, in the case of bodies made of carbon derived from a preoxidized PAN precursor, it is common practice to perform two successive stages:                a first stage of carbonization proper in which the precursor is chemically transformed into carbon, this first stage being performed on an industrial scale in an oven by progressively raising the heating temperature of the oven up to about 900° C.; and        a second stage of heat treatment at high temperature seeking in particular to eliminate by sublimation any sodium coming from the precursor, this second stage likewise being performed in an oven by progressively raising its temperature up to about 1600° C., or indeed about 2000° C. to 2200° C., or even 2500° C. when seeking to eliminate other metallic impurities or to perform very high temperature heat treatment on the carbon fibers.        
The second stage is generally performed under low pressure while sweeping with an inert gas such as nitrogen.
When the carbon bodies are constituted by reinforcing fiber fabric for parts made of composite material, the second stage is generally performed prior to densifying the fiber fabric with the resin, carbon, or ceramic matrix of the composite material. For a thermostructural composite material having a matrix made of carbon and/or ceramic, densification can be performed by a liquid method, i.e. by impregnation with a liquid compound such as a resin that constitutes a precursor for the material of the matrix, and then by transforming the precursor by means of heat treatment. Densification can also be performed by a gaseous method, i.e. by chemical vapor infiltration, where both these methods, the liquid method and the gaseous method, are well known and may optionally be used in association with each other.
In existing installations, the cooling of the gaseous effluents leads to a deposit containing sodium being formed on the walls of the pipes downstream from the outlet for effluent leaving the heat treatment oven. It is necessary to clean these pipes regularly, and such cleaning is not easy because of the risk of the sodium-containing deposit reacting violently.